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The Coanda effect is the phenomenon in which a jet flow attaches
itself to a nearby surface and remains attached even when the
surface curves away from the initial jet direction. The phenomenon
derives its name from a Romanian born aeronautical engineer -
Henri Coanda.
In free surroundings, a jet of fluid entrains and mixes with
its surroundings as it flows away from a nozzle.
When a surface is brought close to the jet, it restricts the
entrainment in that region. As flow accelerates to try balance
the momentum transfer, a pressure difference across the jet results
and the jet is deflected closer to the surface - eventually attaching
to it.
JET ATTACHES TO ADJACENT HULL SURFACE
Even if the surface is curved away from the initial direction,
the jet tends to remain attached. This effect can be used to
change the jet direction. In doing so, the rate at which the
jet mixes is often significantly increased compared with that
of an equivalent free jet.
The jet stream exiting the nozzle of a thruster is affected
by the Coanda effect, since the bottom of the hull is an adjacent
flat surface. When a conventional azimuth thruster on a semi-submersible
vessel is thrusting in the athwart ship direction, toward the
opposing pontoon, the jet stream will first attach itself to
the bottom of the pontoon where the thruster is installed. It
will then follow the radius at the edge of the pontoon, turning
the jet stream in an upward direction. It will then cross the
distance toward the opposing pontoon where it hits the side of
that pontoon.
JET ATTACHES TO THE BOTTOM OF PONTOON, TURNS
UPWARD AT RADIUS ON PONTOON CORNER AND HITS THE OPPOSING PONTOON
This results in two significant thrust losses:
1. Friction of the jet stream where it flows along the bottom
plating of the pontoon at high velocity.
2. Pressure of the jet stream exerted on the opposing pontoon.
Modeling using CFD analysis (Computational Fluid Dynamics)
and in model test basins have shown that these losses can be
as high as 46% of the total net thrust produced by the propeller
and nozzle.
The jet stream exiting the nozzle is diverging at a total
included angle of approximately 13 degrees. Through CFD analysis,
Thrustmaster has discovered that when the jet stream is directed
at an angle of not less than seven degrees away from the hull
plating, it will no longer tend to attach itself to the hull
plating. The jet will completely separate itself from the hull
and will continue to move away from it. Based on this knowledge,
Thrustmaster developed azimuth thrusters with a 7 degree downward
thrust angle. These thrusters use 97 degree gears instead of
the conventional 90 degree gears. This eliminates the thrust
losses described above.
In addition to this, the 7 degree down angle also eliminates
interaction between thrusters. The jet stream from any thruster
will no longer affect other thrusters when that jet stream is
directed toward that other thruster. There are no "forbidden
zones" required in the DP program. This substantially improves
the station keeping capability of the vessel.
Some of the other thruster manufacturers have tried to compensate
for the Coanda effect by using their standard 90 degree geared
thrusters and tilting the nozzle downward. Tilting of the nozzle
creates undesirable side effects, such as very high clearances
between propeller blade tips and nozzle in the top and bottom
locations, uneven propeller inflow velocities, uneven pressure
distribution across propeller and nozzle disc areas, etc. Most
importantly, the maximum tilting angle is about 5 degrees, which
is insufficient to avoid the Coanda effect, so tilting the nozzle
only is ineffective in avoiding the thrust losses described above.
For semi-submersible drilling rigs, Thrustmaster thrusters
with a 7 degree down angle are much more efficient than any other
thrusters.
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